This application claims the priority of German Application Ser. No. 197 48 608.8, filed Nov. 4, 1997, which is incorporated herein by reference.
The invention relates to a sampling Phase Locked Loop (PLL) for high-resolution radar systems, having a controllable frequency oscillator, whose output signal is partially coupled out into a feedback branch and compared with reference frequencies in a phase comparator, for whose generation at least one quartz oscillator is provided.
In connection with high-resolution radar systems, it is often not possible to employ PLLs with a frequency divider in the feedback branch. The divider noise occurring in such PLLs too greatly impairs the frequency stability of the output signal.
In another known embodiment variant of a PLL which offers an improvement in regard to phase noise at the PLL output, the phase comparison is performed on a defined harmonic of the reference signal. The increased technical outlay for this variant makes possible the transfer of the quartz oscillator stability, which can be realized, to the output signal of the PLL even for the frequency multiplications. In this variant the selection of a defined upper harmonic wave, in particular in connection with large multiples of the reference frequency, takes place indirectly by the PLL. The latter can be considered to be a bandpass filter with a bandwidth corresponding to the bandwidth of the control loop. The selection of a defined frequency is performed by a pre-tuning process which takes place, for example, by a conventional digital control loop with a frequency divider described at the outset. When using a YIG oscillator with a digital driver, the preselection can take place by means of the driver. The actual control of the PLL takes place via the fine tuning of the YIG oscillator.
The reference frequency of such a sampling PLL is determined by the required step size. Therefore very large multiplication factors at the reference oscillator are required for generating high output frequencies. Besides the problem of achieving a sufficiently high output yield at high multiples of the reference frequency, which is necessary for maintaining a sufficient distance from the thermal noise, each doubling of the reference frequency raises the phase noise by 6 dB. Therefore, even with this concept the frequency stability is impaired, if high output frequencies are demanded with small step sizes in high-resolution radar systems.
It therefore is the object of the invention to provide a low-noise sampling PLL for high-resolution radar systems.
The above object generally is achieved according to the present invention by a sampling PLL for high-resolution radar systems, which has a controllable frequency oscillator, whose output signal is partially coupled out into a feedback branch and compared with reference frequencies in a phase comparator, with the reference frequencies being taken from a line spectrum (SPK), which has been generated from the mixture of a low quartz oscillator frequency (F1) with a high quartz oscillator frequency (F2), and with the quartz oscillator frequencies (F1, F2) being synchronized.
Further embodiments and modification of the invention are disclosed.
The invention makes advantageous use of a property of quartz oscillators which is, that it is possible to realize quartz oscillators with approximately the same phase noise performance in a frequency range between 5 MHZ to 100 MHZ. Based on this property, in accordance with the invention, the reference signal for the sampling PLL is generated in the form of a line spectrum by means of frequency mixing. It is possible in this way to generate high output frequencies with small multiplication factors. When using the attainment of the object in accordance with the invention, a clear improvement in stability of the output signal occurs because of the given square dependency of the phase noise on the multiplication factors.